There is strong evidence for an association between regular physical activity and reduced risk for cancers of the breast and colon. The majority of adults do not engage in enough physical activity, and rates among overweight adults are particularly low. Overweight status is associated with an increased risk of multiple other cancers, including endometrial, liver, esophageal, and lung cancer. Motivating overweight adults to exercise is critical to cancer prevention efforts. However, additional work is needed to improve the theoretical frameworks applied to exercise promotion. The most often cited theories of health promotion include the outcomes of a target behavior as important determinants. The perceptions people have about the outcomes of exercise, and more generally attitudes about exercise, are associated with exercise participation. Conceptualizations of perceived outcomes are categorized by whether they are instrumental (i.e. utility-based) or affective (i.e. feeling-based) in nature. Recent efforts to compare the relative predictive power of instrumental and affective attitudes suggest that affective attitudes may better predict exercise behavior. Aim 1 of this proposal seeks to compare the relative influence of instrumental versus affective perceptions (i.e., attitudes and outcome expectancies) on exercise behavior. Another distinction can be made between perceptions that are held temporally proximal versus distal to exercise behavior. In the vast majority of relevant research in the exercise field, perceptions are assessed and then future exercise behavior is assessed months later via self-report. However, day to day perceptions and exercise behavior is largely unknown. The temporal distance of perceptions from the decision to exercise can be significantly shortened using ecological momentary assessments. Aim 2 of this proposal seeks to compare the relative predictive power of temporally distal versus proximal perceptions on exercise behavior. Finally, aim 3 will examine the interaction between instrumental/affective and temporally proximal/distal perceptions (i.e., attitudes and outcome expectancies) for predicting exercise behavior. The proposed research seeks to contribute to cancer prevention efforts by examining underlying attitudes and outcome expectancies that motivate the day to day decision to exercise among overweight adults. In particular, technological advances in mobile platforms to deliver interventions to people wherever they are requires better support for how to apply these methods. Using theory-based, empirically supported concepts, this longitudinal study will follow overweight adults over 12 weeks using electronic diaries to measure their perceptions and exercise behavior change while they receive an exercise intervention. This study will provide a fine-grained examination of the determinants of exercise targeted in exercise promotion interventions. The results will inform future efforts to promote exercise using mobile technologies by determining what types of beliefs (instrumental/affective) and what times (temporal proximity to behavior) are most critical times to intervene.